1. Field of the Invention
The present invention relates to a giant magnetoresistive (GMR) angle sensor for vehicles that includes a GMR element showing a significant change in resistance in response to a change in an external magnetic field.
2. Description of the Related Art
A GMR angle sensor is an angle sensor including a giant magnetoresistive element (GMR element) whose resistance significantly changes in response to a change in an external magnetic field. As is generally known, such a GMR element has a laminated structure of free magnetic layer/nonmagnetic conductive layer/pinned magnetic layer/antiferromagnetic layer. The magnetization direction of the pinned magnetic layer is pinned in one direction by an exchange coupling magnetic field generated at the interface with the antiferromagnetic layer. The magnetization direction of the free magnetic layer facing the pinned magnetic layer, with the nonmagnetic conductive layer provided therebetween, is rotated in response to an external magnetic field, and the resistance is changed in accordance with the angle formed by the magnetization direction of the pinned magnetic layer and the magnetization direction of the free magnetic layer. This GMR angle sensor includes an electrode layer connected to either end of the GMR element. When a certain amount of current is provided to the GMR element through the electrode layer, a change in resistance of the GMR element is detected as a change in voltage. The GMR element and the electrode layer (except for electrode pads) are sealed with a protective layer, and the protective layer is generally composed of an inorganic film such as a SiO2 film.
Since the above GMR angle sensor can detect angles without being in contact with a movable portion (rotating portion), reliability and durability thereof are significantly higher than those of a contact-type angle sensor using sliding resistance. Therefore, application of the GMR angle sensor to an angle sensor for vehicles, for example, for detecting the steering angle of a steering wheel has been expected.
It is assumed that GMR angle sensors for vehicles are used in an environment in which the temperature varies over a wide range of at least about −40 to 160 degrees Celsius. Therefore, it is desired that the output characteristics of a GMR element do not fluctuate with change in the temperature in the operating environment, that is, the value of element resistance of the GMR element is stable regardless of the temperature in the operating environment. Examples of patent documents in the related art include Japanese Unexamined Patent Application Publication Nos. 8-70148, 8-264861, 11-287669, 2002-107433, 2000-213957, 2000-180524, and 7-63505.
However, although an inorganic film such as a SiO2 film, which is used in known sensors, has excellent heat resistance, such an inorganic film does not satisfactorily meet the levels of moisture resistance and corrosion resistance required in the environment in which GMR sensors for vehicles operate. Therefore, in order to improve moisture resistance and corrosion resistance, it is believed that the use of an organic film is necessary.
For example, Japanese Unexamined Patent Application Publication Nos. 11-287669, 2002-107433, and 7-63505 describe that an organic film may be used as a protective film provided on a wafer. In these patent documents, a polyimide resin is used for the organic film.
However, since the curing temperature of polyimide resins is high, i.e., 300° C. or higher, if a polyimide resin were used to protect a GMR element, the GMR element would be exposed to a high temperature during curing of the resin after the formation of the film. Accordingly, characteristics of the GMR element may be degraded, and thus polyimide resins cannot be used to form protective films for GMR elements.
In order to improve heat resistance, moisture resistance, corrosion resistance, adhesiveness, and the like, not only the type of material of the protective film but also the structure of the protective film is important. However, the above patent documents do not describe the structure of such protective films in detail.